This invention relates generally to a timing signal referenced clock and is particularly directed to a clock periodically updated by a received reference timing signal, and wherein the clock's internal timing circuit is corrected for more accurate operation in between the periodic clock updates.
Atomic resonance can be used to provide time scales having a high degree of uniformity and reproducibility. Thus, an atomic clock with a cesium-atom oscillator loses only one second every 370,000 years and is thus stable to one part in 10.sup.12. Despite this high degree of accuracy, atomic-based units of time are not used for general measurement purposes as the occurrence of events are generally measured and recorded in terms of solar, or sidereal, time. This time scale is based upon the mean time of rotation of the earth about its axis in relation to the vernal-equinox point in the sky. It is determined by observing the meridian transits of stars. The mean sidereal day is 23 hours, 56 minutes, and 4.09 seconds. Because of variations in the rotational speed of the earth, sidereal time is not perfectly uniform.
A universal time scale, also known as Greenwich Mean Time or Universal Coordinated Time (UTC), is based on the mean angle of rotation of the earth about its axis in relation to the sun. It is referenced to the prime meridian that passes through Greenwich, England. In UTC, an atomic clock provides the basic intervals of time and, when necessary, corrections are made to keep the clock in agreement with solar time. In this way, UTC maintains accurate solar time while providing the time-interval precision required by many sciences and businesses, such as in the fields of astronomy and communications. For practical purposes, UTC can be considered as solar time measured at the prime meridian (Greenwich Mean Time), but the basic intervals of UTC are counted by the much more precise atomic clock.
In order to provide a worldwide time reference signal, various radio frequency (RF) transmitter stations synchronized with a master standard atomic clock maintained by the Bureau International de L'Heure (BIH) in Paris are located throughout the world. The National Bureau of Standards (NBS) provides the standard reference time for the United States by broadcasts from standard frequency and time stations in Colorado (WWV) and Hawaii (WWVH). The regular (WWV) carrier at 2.5, 5, 10, 15 and 20 MHz uses a 1000 Hz amplitude modulating tone burst to signal the beginning of each minute. WWVH uses a 1200 Hz amplitude modulating tone burst. A 100 Hz subcarrier contains binary coded decimal (BCD) signals that supply day-of-the-year, hour and minute information. Complete BCD information in the form of a frame is transmitted each minute. This information is encoded by controlling the width of the 1-second subcarrier pulses. This BCD time information is updated every minute. Also contained in the BCD signals are UTl data which provides a correction for periodic variations in the speed of rotation of the earth and normal/daylight saving time data.
To date, laboratory reference oscillators for receiving the aforementioned time-based signals for use as frequency standards are available. However, there is not presently known a clock capable of receiving and displaying the time provided by the aforementioned WWV and WWVH time-based signals. The present invention is therefore intended to provide a relatively low cost, easily installed and aligned clock which is capable of receiving UTC time signals on any one of three frequencies, displaying the time to an accuracy of .+-.10 milliseconds, and regularly and continuously updating the timing of a microprocessor used to control the clock. The present invention is not subject to the timing inaccuracies inherent in a temperature-stabilized crystal oscillator circuit nor does it exhibit the cumulative time error inherent in atomic clocks.